There is an ongoing effort to develop systems that are more energy-efficient. A large proportion (some estimates are as high as twenty-five percent) of the electricity generated in the United States each year goes to lighting, a large portion of which is general illumination (e.g., downlights, flood lights, spotlights and other general residential or commercial illumination products). Accordingly, there is an ongoing need to provide lighting that is more energy-efficient.
Persons of skill in the art are familiar with, and have ready access to, a wide variety of types of light sources, and any suitable light source (or light sources) can be employed in lighting devices in accordance with the present inventive subject matter.
Representative examples of types of light sources include incandescent lights, fluorescent lamps, solid state light emitters, laser diodes, thin film electroluminescent devices, light emitting polymers (LEPs), halogen lamps, high intensity discharge lamps, electron-stimulated luminescence lamps, etc., with or without filters. That is, lighting devices in accordance with the present inventive subject matter can comprise a single light source, a plurality of light sources of a particular type, or any combination of one or more light sources of each of a plurality of types.
Persons of skill in the art are also familiar with, and have ready access to, a wide variety of light sources (of any type or combination of types) that emit light of any hue, and any suitable hue-emitting light source (or light sources), or combination of hue-emitting light sources, can be employed in lighting devices in accordance with the present inventive subject matter. While there is a need for lighting devices that provide more efficient white lighting, there is in general a need for lighting devices that provide more efficient lighting in all hues.
In some aspects, the present inventive subject matter is directed to lighting devices that comprise one or more solid state light emitters (e.g., one or more LEDs and/or one or more luminescent materials). While there is much discussion herein of the merits of solid state light emitters, many aspects of the present inventive subject matter as discussed herein can, if desired, be applied to lighting devices that comprise other types of light sources (rather than or in addition to one or more solid state light emitters), e.g., incandescent light sources, fluorescent light sources, etc. Similarly, while there is much discussion herein of lighting devices that emit white light, the present inventive subject matter is applicable to lighting devices that emit light of any desired hue.
Solid state light emitters (e.g., light emitting diodes) are receiving much attention due to their energy efficiency. It is well known that incandescent light bulbs are very energy-inefficient light sources—about ninety percent of the electricity they consume is released as heat rather than light. Fluorescent light bulbs are more efficient than incandescent light bulbs (by a factor of about 10), but are still less efficient than solid state light emitters, such as light emitting diodes.
In addition, as compared to the normal lifetimes of solid state light emitters, e.g., light emitting diodes, incandescent light bulbs have relatively short lifetimes, i.e., typically about 750-1000 hours. In comparison, light emitting diodes, for example, have typical lifetimes between 50,000 and 70,000 hours. Fluorescent light bulbs have longer lifetimes than incandescent light bulbs (e.g., fluorescent bulbs typically have lifetimes of 10,000-20,000 hours), but provide less favorable color reproduction. The typical lifetime of conventional fixtures is about 20 years, corresponding to a light-producing device usage of at least about 44,000 hours (based on usage of 6 hours per day for 20 years). Where the light-producing device lifetime of the light source (or light sources) is less than the lifetime of the fixture, the need for periodic change-outs is presented. The impact of the need to replace light sources is particularly pronounced where access is difficult (e.g., vaulted ceilings, bridges, high buildings, highway tunnels) and/or where change-out costs are extremely high.
General illumination lamps are typically rated in terms of their color reproduction. Color reproduction is typically measured using the Color Rendering Index (CRI Ra). CRI Ra is a modified average of the relative measurements of how the color rendition of a lamp compares to that of a reference radiator when illuminating eight reference colors, i.e., it is a relative measure of the shift in surface color of an object when lit by a particular lamp. The CRI Ra equals 100 if the color coordinates of a set of test colors being illuminated by the lamp are the same as the coordinates of the same test colors being irradiated by the reference radiator.
Daylight has a high CRI (Ra of approximately 100), with incandescent bulbs also being relatively close (Ra greater than 95), and fluorescent lighting being less accurate (typical Ra of 70-80). Certain types of specialized lighting have very low CRI (e.g., mercury vapor or sodium lamps have Ra as low as about 40 or even lower). Sodium lights are used, e.g., to light highways—driver response time, however, significantly decreases with lower CRI Ra values (for any given brightness, legibility decreases with lower CRI Ra). The color of visible light output by a light source, and/or the color of blended visible light output by a plurality of light emitters can be represented on either the 1931 CIE (Commission International de I'Eclairage) Chromaticity Diagram or the 1976 CIE Chromaticity Diagram. Persons of skill in the art are familiar with these diagrams, and these diagrams are readily available (e.g., by searching “CIE Chromaticity Diagram” on the internet).
The CIE Chromaticity Diagrams map out the human color perception in terms of two CIE parameters x and y (in the case of the 1931 diagram) or u′ and v′ (in the case of the 1976 diagram). Each point (i.e., each “color point”) on the respective Diagrams corresponds to a particular hue. For a technical description of CIE chromaticity diagrams, see, for example, “Encyclopedia of Physical Science and Technology”, vol. 7, 230-231 (Robert A Meyers ed., 1987). The spectral colors are distributed around the boundary of the outlined space, which includes all of the hues perceived by the human eye. The boundary represents maximum saturation for the spectral colors.
The 1931 CIE Chromaticity Diagram can be used to define colors as weighted sums of different hues. The 1976 CIE Chromaticity Diagram is similar to the 1931 Diagram, except that similar distances on the 1976 Diagram represent similar perceived differences in color.
The expression “hue”, as used herein, means light that has a color shade and saturation that correspond to a specific point on a CIE Chromaticity Diagram, i.e., a point that can be characterized with x,y coordinates on the 1931 CIE Chromaticity Diagram or with u′, v′ coordinates on the 1976 CIE Chromaticity Diagram.
In the 1931 Diagram, deviation from a point on the Diagram (i.e., “color point”) can be expressed either in terms of the x, y coordinates or, alternatively, in order to give an indication as to the extent of the perceived difference in color, in terms of MacAdam ellipses. For example, a locus of points defined as being ten MacAdam ellipses from a specified hue defined by a particular set of coordinates on the 1931 Diagram consists of hues that would each be perceived as differing from the specified hue to a common extent (and likewise for loci of points defined as being spaced from a particular hue by other quantities of MacAdam ellipses).
A typical human eye is able to differentiate between hues that are spaced from each other by more than seven MacAdam ellipses (but is not able to differentiate between hues that are spaced from each other by seven or fewer MacAdam ellipses).
Since similar distances on the 1976 Diagram represent similar perceived differences in color, deviation from a point on the 1976 Diagram can be expressed in terms of the coordinates, u′ and v′, e.g., distance from the point=(Δu′2+Δv′2)1/2. This formula gives a value, in the scale of the u′ v′ coordinates, corresponding to the distance between points. The hues defined by a locus of points that are each a common distance from a specified color point consist of hues that would each be perceived as differing from the specified hue to a common extent.
A series of points that is commonly represented on the CIE Diagrams is referred to as the blackbody locus. The chromaticity coordinates (i.e., color points) that lie along the blackbody locus obey Planck's equation: E(λ)=Aλ−5/(e(B/T)−1), where E is the emission intensity, λ is the emission wavelength, T is the color temperature of the blackbody and A and B are constants. The 1976 CIE Diagram includes temperature listings along the blackbody locus. These temperature listings show the color path of a blackbody radiator that is caused to increase to such temperatures. As a heated object becomes incandescent, it first glows reddish, then yellowish, then white, and finally blueish. This occurs because the wavelength associated with the peak radiation of the blackbody radiator becomes progressively shorter with increased temperature, consistent with the Wien Displacement Law. Illuminants that produce light that is on or near the blackbody locus can thus be described in terms of their color temperature.
The most common type of general illumination is white light (or near white light), i.e., light that is close to the blackbody locus, e.g., within about 10 MacAdam ellipses of the blackbody locus on a 1931 CIE Chromaticity Diagram. Light with such proximity to the blackbody locus is referred to as “white” light in terms of its illumination, even though some light that is within 10 MacAdam ellipses of the blackbody locus is tinted to some degree, e.g., light from incandescent bulbs is called “white” even though it sometimes has a golden or reddish tint (e.g., light having a correlated color temperature of 1500 K or less is reddish).
Light emitting diodes are increasingly being used in lighting/illumination applications, such as traffic signals, color wall wash lighting, backlights, displays and general illumination, with one ultimate goal being a replacement for the ubiquitous incandescent light bulb.
The emission spectrum of any particular light emitting diode is typically concentrated around a single wavelength (as dictated by the light emitting diode's composition and structure), which is desirable for some applications, but not desirable for others, (e.g., for providing general illumination, such a narrow emission spectrum would, by itself, provide a very low CRI Ra).
Light that is perceived as white can be made by blending two or more colors (or wavelengths). “White” solid state light emitting lamps have been produced by providing devices that mix different colors of light, e.g., by using light emitting diodes that emit light of differing respective colors and/or by converting some or all of the light emitted from the light emitting diodes using luminescent material. For example, as is well known, some lamps (referred to as “RGB lamps”) use red, green and blue light emitting diodes, and other lamps use (1) one or more light emitting diodes that generate blue light and (2) luminescent material (e.g., one or more phosphor materials) that emits yellow light in response to excitation by blue light emitted by the light emitting diode, whereby the blue light and the yellow light, when mixed, produce light that is perceived as white light.
In order to provide a broad spectrum light source (such as a white light source) in a lamp that comprises a relatively narrow spectrum light source (such as a light emitting diode) the relatively narrow spectrum of the light emitting diode may be shifted and/or spread in wavelength using one or more luminescent materials. For example, a “white” LED may be formed by coating a light emitting diode (e.g., one that emits blue light) with an encapsulant material, such as a resin or silicon, that includes therein a wavelength conversion material, such as a YAG:Ce phosphor, that emits yellow light in response to stimulation with blue light. Some, but not all, of the blue light that is emitted by the light emitting diode is absorbed by the phosphor, causing the phosphor to emit yellow light. The blue light emitted by the light emitting diode that is not absorbed by the phosphor combines with the yellow light emitted by the phosphor, to produce light that is perceived as white by an observer. Other combinations also may be used. For example, a red emitting phosphor can be mixed with a yellow phosphor to produce light having a different color temperature and/or better color rendering properties. Alternatively, one or more light emitting diodes that emit red light may be used to supplement light emitted by a blue light-emitting light emitting diode that is coated with a yellow light-emitting phosphor. In other alternatives, separate red, green and blue light emitting diodes may be used. Moreover, infrared (IR) or ultraviolet (UV) light emitting diodes may be used. Finally, any or all of such combinations (or other combinations) may be used analogously to produce hues other than white.
Lamps that comprise one or more solid state light emitters can offer a long operational lifetime relative to conventional incandescent and fluorescent bulbs. Lifetime of lamps that comprise one or more solid state light emitters is typically measured by an “L70 lifetime”, i.e., a number of operational hours in which the light output of the lamp does not degrade by more than 30%. Typically, an L70 lifetime of at least 25,000 hours is desirable, and has become a standard design goal. As used herein, L70 lifetime is defined by Illuminating Engineering Society Standard LM-80-08, entitled “IES Approved Method for Measuring Lumen Maintenance of LED Light Sources”, Sep. 22, 2008, ISBN No. 978-0-87995-227-3, also referred to herein as “LM-80”, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein.
Various embodiments are described herein with reference to “expected L70 lifetime.” Because the lifetimes of lamps that comprise one or more solid state light emitters are typically measured in the tens of thousands of hours, it is generally impractical to perform full term testing to measure the lifetime of the product. Therefore, projections of lifetime from test data on the system and/or light source are used to project the lifetime of the system. Such testing methods include, but are not limited to, the lifetime projections found in the ENERGY STAR Program Requirements cited above or described by the ASSIST method of lifetime prediction, as described in “ASSIST Recommends . . . LED Life For General Lighting: Definition of Life”, Volume 1, Issue 1, February 2005, the disclosure of which is hereby incorporated herein by reference as if set forth fully herein. Accordingly, the term “expected L70 lifetime” refers to the predicted L70 lifetime of a product as evidenced, for example, by the L70 lifetime projections of ENERGY STAR, ASSIST and/or a manufacturer's claims of lifetime.
Solid state light emitters, such as light emitting diodes or LEDs, may be energy efficient, so as to satisfy ENERGY STAR® program requirements. ENERGY STAR program requirements are defined in “ENERGY STAR® Program Requirements for Solid State Lighting Luminaires, Eligibility Criteria—Version 1.1”, Final: Dec. 19, 2008, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein.
In order to encourage development and deployment of highly energy efficient solid state lighting (SSL) products to replace several of the most common lighting products currently used in the United States, including 60-watt A19 incandescent and PAR 38 halogen incandescent lamps, the Bright Tomorrow Lighting Competition (L Prize™) has been authorized in the Energy Independence and Security Act of 2007 (EISA). The L Prize is described in “Bright Tomorrow Lighting Competition (L Prize™)”, May 28, 2008, Document No. 08NT006643, the disclosure of which is hereby incorporated herein by reference in its entirety as if set forth fully herein. The L Prize winner must conform to many product requirements including light output, wattage, color rendering index, correlated color temperature, expected lifetime, dimensions and base type.